Acute retinal or optic nerve head damage, which can result in the loss of vision, is caused by trauma and various pathological events such as ischemia, hypoxia, or edema.
Retinal or optic nerve head ischemia or hypoxia results when blood supply is significantly reduced to these tissues. Ischemia is a complex pathological episode involving numerous biochemical events. In recent years, the involvement of excitatory amino acids in ischemia-related neuronal and retinal damage has been implicated. (See Choi, Excitatory cell death, Journal of Neurobiology, volume 23, pages 1261-1276 (1992); Tung et al., A quantitative analysis of the effects of excitatory neurotoxins on retinal ganglion cells in the chick, Visual Neuroscience, volume 4, pages 217-223 (1990); Sisk et al., Histologic changes in the inner retina of albino rats following intravitreal injection of monosodium L-glutamate, Graefe's Archive for Clinical and Experimental Ophthalmology, volume 223, pages 250-258 (1985); Siliprandi et al., N-methyl-D-aspartate-induced neurotoxicity in the adult rat retina, Visual Neuroscience, volume 8, pages 567-573 (1992); and David et al., Involvement of excitatory neurotransmitters in the damage produced in chick embryo retinas by anoxia and extracellular high potassium, Experimental Eye Research, volume 46, pages 657-662 (1988)) During ischemia or hypoxia, excitatory amino acids are markedly elevated (Benveniste et al, Elevation of the extracellular concentrations of glutamate and aspartate in rat hippocampus during transient cerebral ischemia monitored by intracerebral microdialysis, Journal of Neurochemistry, volume 43, pages 1369-1374 (1984)), the consequences of which may lead to excessive stimulation of post-synaptic excitatory amino acid receptors, and potentially resulting in cell injury. Antagonists against excitatory amino acid receptors have been shown to reduce neuronal and retinal damage in ischemic conditions. (See Sheardown et al., 2,3-Dihydorxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline: a neuroprotectant for cerebral ischemia, Science, volume 247, pages 571-574 (1990); Scatton et al., Eliprodil Hydrochloride, Drugs of the Future, volume 19, pages 905-909 (1994); and Sucher et al., N-methyl-D-aspartate antagonists prevent kainate neurotoxicity in rat retinal ganglion cells in vitro, Journal of Neuroscience, volume 11, pages 966-971 (1991)). Release of excitatory amino acids has been demonstrated to cause cytotoxicity due to increases in intracellular calcium levels, which in turn affects protein phosphorylation, proteolysis, lipolysis, and ultimately causing cell death. (See Choi, Glutamate neurotoxicity and diseases of the nervous system, Neuron, volume 1, pages 623-634 (1988); Siesjo, Calcium, excitotoxins, and brain damage, NIPS, volume 5, pages 120-125 (1990) and Olney et al., The role of specific ions in glutamate neurotoxicity, Neuroscience Letters, volume 65, pages 65-71 (1986)). The elevation of intracellular levels of calcium occurs in part by excitatory amino acid-induced depolarization of the cell membrane and subsequent activation of post-synaptic voltage-dependent calcium channels.
Hence, inhibitors of post-synaptic voltage-dependent calcium channels including the dihydropyridines, such as nitrendipine and nifedipine; the phenylalkylamines, such as verapamil, the diphenylalkylamines, such as flunarizine; and the benzothiazepines, such as diltiazem, were also effective in the protection of neuronal damage after ischemia (Deshpande et al., Flunarizine, a calcium entry blocker, ameliorates ischemic brain damage in the rat, Anethesiology, volume 64, pages 215-224 (1986); and Sauter et al., Treatment of hypertension with isradipine reduces infarct size following stroke in laboratory animals, American Journal of Medicine, volume 86, pages 130-133 (1989)). Unfortunately, these same agents may block voltage-dependent calcium channels in vascular smooth muscles, potentially leading to vasodilation and possibly systemic hypotension.
A new development in the treatment of ischemia-induced neuronal damage is to minimize the release of excitatory amino acids from the pre-synaptic nerve terminal. This release is dependent upon an elevation of calcium in the nerve terminal. The pre-synaptic calcium influx into neuronal tissues is believed to be mediated by the N-type calcium channel, which can be selectively inhibited by .omega.-conotoxins (Reynolds et al., Brain voltage sensitive calcium channel subtypes differentiated by .omega.-conotoxin fraction GVIA, Proceedings of the National Academy of Science, USA, volume 83, pages 8804-8807 (1986); and McCleskey et al., .omega.-Conotoxin: Direct and persistant blockade of specific types of calcium channels in neurons but not muscle, Proceedings of the National Academy of Science, USA, volume 84, pages 4327-4331 (1987)). Indeed, treatment of animals with .omega.-conotoxins protects them against ischemic damage (Smith et al., Postischemic treatment with the .omega.-conopeptide SNX 111 protects the rat brain against ischemic damage, Fourth International Symposium in Pharmacology of Cerebral Ischemia, volume 36, pages 161-166 (1992); and Valentino et al., A selective N-type calcium channel antagonist protects against neuronal loss after global cerebral ischemia, Proceedings of the National Academy of Science, USA, volume 90, pages 7894-7897 (1993)).
The conotoxins are a class of small peptides derived from the molusk genus Conus. Over 500 species of this genus have been reported to exist (U.S. Pat. No. 5,432,155, Olivera et al.). The conotoxins of the C. geographus and C. magus species have been particularly studied. The conotoxins are further classified by their biological activity. For example, .alpha.-conotoxins block the nictotinic acetylcholine receptor, .mu.-conotoxins block skeletal muscle sodium channels, and .omega.-conotoxins block pre-synaptic neuronal calcium channels (Cruz, Conus Venoms: A Rich Source of Neuroactive Peptides, Journal of Toxicology and Toxin Review, volume 4, pages 107-132 (1985)).
.omega.-conotoxins have been researched extensively in the area of N-type pre-synaptic voltage activated calcium channels. These channels are distributed predominantly in neuronal cells. The following publications may be referred to for further background and characterization of the effects of .omega.-conotoxins on voltage activated calcium channels in neuronal tissue:
Sher et al., .omega.-Conotoxin Binding and Effects on Calcium Channel Function in Human Neuroblastoma and Rat Pheochromocytoma Cell Lines, FEBS Letters, volume 235, number 1,2, pages 178-182(1988);
Cruz et al., Characterization of the .omega.-Conotoxin Target. Evidence For Tissue-Specific Hetereogeneity in Calcium Channel Types, Biochemistry, volume 26, pages 820-824 (1987); and
Jones et al., Localization and Mobility of .omega.-conotoxin-Sensitive Ca.sup.2+ Channels in Hippocampal CA1 Neurons, Science, volume 244, pages 1189-1193 (1989).
At least 11 homologous .omega.-conotoxins have been discovered. A number of publications disclose these conotoxins including: WIPO Publication No. WO 93/13128, for use as cerebral analgesics; U.S. Pat. No. 5,189,020 (Miljanich et al.), for brain ischemia therapy; and 4,950,739 (Cherksey et al.), for use as agents for blocking, isolating and purifying of calcium channels. Nowhere in the art, however, has it been proposed to acutely administer .omega.-conotoxins to prevent or ameliorate retinal or optic nerve head damage.